1. Field of the Invention
This invention relates to a dielectrically-loaded antenna for operation at frequencies in excess of 200 MHz.
2. Discussion of the Related Art
Such antennas are disclosed in a number of patent publications of the present applicant, including GB2292638A, GB2309592A, GB2310543A, GB2338605A, GB2346014A GB2351850A and GB2367429A. Each of these antennas has at least one pair of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a material having a relative dielectric constant greater than 5. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore containing a coaxial feed structure comprising an inner conductor surrounded by a shield conductor. At one end of the core the feed structure conductors are connected to respective antenna elements which have associated connection portions adjacent the end of the bore. At the other end of the bore, the shield conductor is connected to a conductor which links the antenna elements and, in each of these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed structure.
Some of the above prior patent publications disclose quadrifilar helical antennas. Each of these antennas has four helical tracks plated on the cylindrical surface of the core, or four groups of helical tracks, each group comprising two tracks separated by a narrow slit. Whether the antenna has four helical tracks or two, the connection portions connecting the antenna elements to the feed structure conductors are radial tracks plated on a planar end surface of the core.
It is known to provide a quadrifilar helical with an impedance matching network. This may be embodied as a printed circuit board depending from the end surface of the core opposite to that bearing the radial connection portions, or it may take the form of a small printed circuit or laminate board secured to the top end face of the core where it provides coupling between the feed structure and radial connection portions such as those disclosed in the above-mentioned prior patent publications. An antenna having such a matching network is disclosed in our co-pending U.S. patent application Ser. No. 11/472,587. The matching network comprises a capacitor connected in parallel across the inner and shield feed conductors, and a series inductance between the inner conductor and the connection portions associated with two of the helical tracks. Connections between the laminate board and the radial connection portions on the end face of the core are made by solder fillets between plated edge portions of the laminate board and the tracks of the radial connection portions, the laminate board lying flat on the core end face.